Originally Posted by major
Not a dumb question. Sure, you could do it. The potentiometer would need to be about ten times the size of your motor. And it would waste half of the battery energy needed for every acceleration. And at any speed less than full speed, it would waste considerable amounts of power.
That's a bit drastic. Let's not scare the poor guy to death!
years, really simple crude low-speed EVs used exactly this type of motor controller. Examples include streetcars, golf carts, fork lifts, and many little personal vehicles. These were serious commercial products made by major companies; not hobby or experimental EVs.
The adjustable resistor in series with the motor is called a "rheostat" (not a potentiometer). At this power level, it is generally built with a number of fixed resistors, selected in by a big rotary switch.
This type of controller was only popular because it was CHEAP! It has no other attributes. It worked best when the vehicle was so under-powered that it was run at full power most of the time. At full power, the resistors are all bypassed and all the power goes to the motor. You ramp the rheostat up from off to full rather quickly (10-20 seconds) to accelerate from a stop to full speed; then cruise at full speed; then ramp the rheostat back down when it's time to stop. Since it's not in use most of the time, efficiency isn't too awfully bad.
Yes, there are still applications for such a thing. For example, I teach kids to build EVs out of old bicycles and other junk they can scrounge (see www.bestoutreach.com
). A typical setup is a car fan motor, with a skateboard wheel pressed onto its shaft, arranged so it rubs against a bike tire to turn it. If you simply switch the motor straight to a 12v battery from a dead stop, the tire slips and quickly burns a hole in the bike tire. But if you put a rheostat in series with the motor, you can slowly ramp up the speed to get it going without slipping. The power of the motor is such that full speed is only 10-15 mph, so you just run it at full speed or off. You can't leave the rheostat at anything but full speed or off for more than 30 seconds or so, or the rheostat overheats.
At this power level, you can buy a proper motor controller for $10 or $20. But a rheostat costs $1 or $2. Guess which one the kids pick?